1. Field of the Invention
The present invention relates to a head up display device for a vehicle.
2. Description of Related Art
Conventionally, a head up display device (hereinafter referred to as an HUD device) for a vehicle is known. The HUD device displays a light-emitting image on a display, and projects the display image onto a projection member such as a windshield. Accordingly, the HUD device displays a virtual image of vehicle-related information. As a type of such a HUD device, a device that reflects a display image of a display on a reflecting mirror such as a concave mirror and projects the reflected image onto a projection member is disclosed in JP-A-2009-132221. As described above, an installation space occupied by the HUD device in a vehicle is made small by the use of the reflecting mirror.
In the HUD device disclosed in JP-A-2009-132221, a configuration, which transmits torque generated, in a motor to the reflecting mirror through a gearing system for adjusting a rotation angle of a rotatably provided reflecting mirror in accordance with an adjustment command from the outside, is employed. As a result of such a configuration, by giving the adjustment command to the HUD device, a vehicle occupant can display a virtual image of vehicle-related information in an easily visually identifiable state.
However, in the HUD device disclosed in JP-A-2009-132221, the last gear of gears in the gearing system, which transmits the torque generated in the motor to the reflecting mirror, includes a partial gear portion having gear teeth arranged in the region in a range of less than 360 degrees in the rotational direction. In the case in which a reduction gear mechanism that amplifies and transmits the torque generated in the motor is employed for reduction of an installation space for the motor due to downsizing of the motor, the greatest torque is applied to the last gear and a gear one step therebefore. Accordingly, in such a reduction gear mechanism, when the generation of torque in accordance with the adjustment command is continued by the motor, despite a stop of rotation of the last gear as a result of the engagement of the gear one step before with a gear tooth of one end of the partial gear portion in the rotational direction, excessive torque which is more than a strength design value may be applied to these gears. Moreover, in a state in which the rotation of the last gear is stopped as a result of the engagement of the gear one step therebefore with the gear tooth of the one end of the partial gear portion in the rotational direction, if the image display does not change despite continuation of the adjustment command by the vehicle occupant, the occupant cannot determine whether the HUD device is at fault, so that decrease of his/her sense of reliability of the device may also be caused.